The image sensor comprises an imager, a telemeter, means for measuring the orientation of its LoS (line of sight) and means for tracking a stationary point on the ground.
The geographical position of an aircraft is not always sufficiently accurate or a fortiori that of the on-board sensor, because for example of inertial drift in the inertial measurement unit of the aircraft, in the absence of GPS. This drift may typically reach 1 nautical mile at the end of 1 h of flight, whereas a positional accuracy much less than 100 m is sought.
The accuracy to which the absolute orientation of the LoS is known is also generally unsatisfactory, i.e. greater than 3 mrd, for example in the following cases:                In the case where the orientation of the LoS of the sensor is obtained from the contribution of the attitude of the aircraft, angles of incidence and feedback of the mechanical angles of the sensor, its accuracy is limited by unknown misalignments between the inertial measurement unit of the aircraft and the LoS of the sensor. Furthermore, these misalignments are in general different in-flight and on the ground, thereby making them difficult to calibrate.        When the optronic sensor is not equipped with an inertial measurement unit (consisting of 3 gyrometers and 3 accelerometers) that could allow, if needs be, its attitude to be determined either autonomously or from inertial alignment with the a priori more accurate attitude of the aircraft. Or, if this is the case, its inertial sensors may be of unsatisfactory class, for reasons of available volume or cost.        
A conventional means for improving the accuracy (“registration”) of the geographical position of a sensor or that of the orientation of its LoS relative to the imprecise orientation and position available, is the use of landmarks. It will be recalled that a landmark is a stationary observable point on the ground that is visible at large distances (water tower, bell tower, antenna, crossroads, bridge, building, etc.) and the geographical coordinates of which are accurately known. However:                The researching and drawing up of lists of usable landmarks requires mission preparation, adding constraints that are not necessarily compatible with unplanned or rapid-response strikes.        The operation/flight required to research/reconnoiter the intended landmark may prove to be resource-intensive.        Simultaneous registration of the position of the sensor and the orientation of its LoS is not possible with a conventional “one-sight” registration on a single landmark, i.e. consisting in exploiting only one sight drawn on the landmark, the measurement then possibly being simply the orientation of the LoS or indeed the orientation-distance vector in the case of telemetry. This single-sight registration allows the orientation of the LoS to be registered in the case where the position of the sensor is known accurately, or the position of the sensor to be registered in the case where the orientation of the LoS (and the distance measured) is sufficiently accurate.        One way of registering in succession the position of the sensor and the orientation of its LoS is to use 2 judiciously placed landmarks, one nearby (for example at 5 km) and the other further away (for example at 30 km). However, practical, accurately and judiciously placed landmarks are not necessarily available in every operational theater.        The accuracy of the registration also depends on the exactitude with which the LoS is targeted on the landmark at the moment at which the measurement is taken. However, for example because of their shape or their context, not all landmarks necessarily lend themselves to being accurately tracked by a LoS.        